1. Field of the Invention
This invention relates to a lens movement control device for a zoom lens having focus adjusting means, suited to a photographic camera, video camera, or television camera.
2. Description of the Related Art
It has been the common practice that the zoom lens has, despite variation of the focal length of the entire system by operating its varifocal section, to keep constant the position of an image plane. In more detail, the varifocal section is constructed from a variator lens unit for varying the focal length of the entire system and a compensator lens unit which compensates for the shift of the image plane resulting from the movement of the variator lens unit. Stability of maintenance of the image plane in the constant position against variation of the focal length of the entire system is thus ensured in the prior art.
In general, to do this, the variator lens unit may be moved linearly on the optical axis, but the compensator lens unit needs to move non-linearly on the optical axis. In most of the zoom lenses, such a non-linear axial movement of the compensator lens unit is controlled mechanically, or by using a cylindrical cam mechanism.
The design of the camming slots for moving the variator lens unit and the compensator lens unit is made so as to assure that their loci of motion give a sharp image on the focal plane in the constant position over the entire zooming range. In fact, in the general case, due to the influence of the tolerances on manufacturing, some focus defects arise with zooming. For example, the refractive index of the medium (glass) of the lens differs from item to item, the lens thickness and the curvature of the lens surface err in machining, and, for the lens barrel, the machining error of the retainer rings and the camming slots change from time to time. For any value of the focal length, therefore, the image of optimum sharpness takes different places (focal plane positions) with different zoom lenses. Such a focal plane deviation due to the manufacturing tolerances is becoming very serious in developing zoom lenses which meet the recent growing demand for a higher zoom ratio, a larger relative aperture and a higher grade imagery.
If this problem is solved by adhering to the conventional type of the operating mechanism for the zoom lens, the production tolerance of each constituent part would have to be made severer, giving an alternative problem that the production cost increases.
In addition, the deficiency of correction of the spherical aberration causes, in many cases, a focal plane deviation whose amount varies with variation of the size of aperture opening.
If this problem is solved by using the conventional method of designing zoom lenses, the spherical aberration would have to be corrected far better than was heretofore possible. This calls for an increase of the number of lens elements, use of expensive materials in some of the lens elements, or taking like measures, giving an alternative problem that the cost of materials increases.
As the related art, there is U.S. Pat. No. 4,740,064 in which the conventional slot type cam ring is not provided and, instead, the mounting and operating mechanism in the barrel gets a complicated structure.